This invention relates to an optical data reproduction apparatus for optically reading data signals recorded onto recording media such as CDs and DVDs and more particularly to wiring patterns or terminal arrangement patterns connected to a light-receiving element.
Optical data reproduction apparatus have light-receiving elements for not only optically reading data signals recorded onto recording media but also detecting tracking and focusing errors. Some light-receiving element has light-receiving cells for detecting a tracking error with three beams, for example, in addition to a known quadrifid light-receiving cell, these light-receiving cells being disposed on one chip. The light-receiving element formed of one chip also has output terminals for leading the signals detected by the respective light-receiving cells to an external circuit, a power supply terminal for introducing a power supply to the light-receiving element, a ground terminal and any other proper terminal. An arrangement of these terminals is determined on the part of manufacturers of light-receiving elements. When the light-receiving element is incorporated into such an optical data reproduction apparatus, it has been attempted to make the order of wiring patterns on a flexible printed board for connection to the external circuit or the order of arranging pins of output connectors correspond to the arrangement of terminals of the light-receiving element.
FIG. 5 shows examples of wiring patterns in a related optical data reproduction apparatus. In FIG. 5, an optical pickup 40 as the optical data reproduction apparatus is connected to the front end IC 52 of a system board 50 via a wiring pattern 46 formed on a flexible printed board. The optical pickup 40 has a light-receiving element 42. The light-receiving element 42 has terminals of E, Vcc, Vc, GND and F in this order on one side and terminals of A, RF, B, C, D, F, E and GND in this order on the other. The light-receiving element 42 is packaged on a wiring board in the optical pickup 40 and connected to connectors for use in external connection via a wiring pattern 44 on the board. The wiring pattern 44 has a jumper line 45 and any other alternative pattern, whereby the connectors are arranged in the order of A, RF, B, C, D, F, E and GND.
A system board 50 also has connectors and a wiring pattern 48 extending from the connectors up to the front end IC 52. An arrangement of connectors on the side of the system board 50 is set conformable to that of connectors on the side of the optical pickup 40. The wiring pattern 46 such as the flexible printed board is used to electrically connect the connectors on the side of the optical pickup 40 and those on the side of the system board 50.
The terminals A, B, C and D of the light-receiving element 42 in the example shown in FIG. 5 are coupled to a quadrifid light receiving cell similar in shape to a quadrifid light-receiving cell 24 of FIG. 2. Output signals from these terminals are used to generate a phase-difference type tracking error signal. A look at the connectors and the wiring patterns shown in FIG. 5 reveals that B, C and D out of the signal lines from the quadrifid light-receiving cell are disposed side by side in this order. In other words, crosstalks are easily produced among the signal lines B, C and D; the drawback to the arrangement above is that a precise tracking error signal is hardly easy to obtain from such a phase-difference system.
A general description will now be given of the generation of a tracking error signal of the phase-difference system together with the reason for the difficulty of obtaining a precise tracking error signal because of crosstalks with reference to FIGS. 6 to 8.
In FIG. 6, reference numeral 54 denotes a quadrifid light-receiving cell. The light-receiving surface of the quadrifid light-receiving cell 54 is divided into four light-receiving cells a, b, c and d. The light-receiving cells a and d, and b and c are orientated in the direction of a track, T, which is equal to the x-axis direction, whereas the light-receiving cells a and b, and c and d are orientated in a direction perpendicular to the direction of the track, which is equal to the y-axis direction. The two light-receiving cells a and c are positioned diagonally, whereas the two light-receiving cells b and d are also positioned diagonally. Outputs of the two diagonal light-receiving cells a and c are added up by an adder 56 as a set so as to obtain an added signal 66. Outputs of the two diagonal light-receiving cells b and d are also added up by an adder 57 as a set so as to obtain an added signal 68.
The added signal 66 is subjected to waveform equalization in a waveform equalizer 58 and also waveform shaping in a waveform shaper 60 before being inputted to a phase comparator 62. Similarly, the added signal 68 is subjected to waveform equalization in a waveform equalizer 59 and also waveform shaping in a waveform shaper 61 before being inputted to the phase comparator 62. In the phase comparator 62, the phases of two sets of outputs thus supplied are compared, so that a pulse signal having width equal to the phase difference between the two sets of outputs. The pulse signal is integrated by a low-pass filter 64 and outputted as a tracking error signal.
The principle of detection by the tracking error detector in the phase difference system will now be described. FIG. 7 refers to a case where a beam spot 70 is moving above the center of the track TC. While the beam spot 70 is related to the pit 72 of a recording medium as shown in FIG. 7A, the dark areas 74 and 76 produced from the light diffraction by the pit 72 within a far field are produced in areas equal to the four light-receiving cells a, b, c and d as shown in FIG. 7B. Therefore, as shown in FIG. 7C, output signal waveforms of the light-receiving cells a, b, c and d are equalized and so are output waveform 66 of the adder 56 and the output waveform 68 of the adder 57 as shown in FIG. 7D. In this state, the phase difference .DELTA.t becomes zero. Here, the variable t is defined by xlv (x: displacement of the beam spot in the track direction T; v: velocity of the beam spot in the track direction T).
Further, FIG. 8A refers to a case where the center of the beam spot SC on the recording medium has been displaced by .DELTA.y in the y-axis direction from the center of the track TC. While the beam spot 70 is related to the pit 72 as shown in FIG. 8A, the dark areas 74 and 76 within the far field appear as shown in FIG. 8B and there is produced differences in the dark areas produced in the four light-receiving cells a, b, c and d, whereby a phase difference of .DELTA.t1 is produced between the output signals of the light-receiving cells a and c and the output signals of the receiving cells b and d, and between the added signals of (a+c) and the added signals of (b+d) as shown in FIGS. 8C and 8D. Consequently, a pulse signal having a pulse width of .DELTA.t1 is produced from the phase comparator 62. This pulse signal is integrated by the low-pass filter 64 to become an output value corresponding to the amount of displacement .DELTA.y, so that tracking control is performed according to the output value.
In accordance with the tracking error detection of the phase difference system as set forth above, in the case of CD-ROM and DVD-ROM drives at from single speed to double speed, for example, output signal frequencies of the light-receiving cells a, b, c and d become considerably high, namely, ranging from 4.5 MHz to 50 several MHz. Consequently, in a case where terminals and wiring patterns B, C and D for passing signals different in phase therethrough are set adjacent to one another as in the related optical data reproduction apparatus shown in FIG. 5, the signal is allowed to leak out of one adjoining wiring pattern and superposed on the signal in the other wiring pattern. Then a so-called crosstalk will occur, thus causing the two signals to mutually damage each other. The action like this will then result in making it difficult to obtain a precise tracking signal. This problem tends to become apparent as the driving speed of the recording medium increases. In the related example of FIG. 5, about 8-times speed is a limit and speed exceeding this limit makes the signal-line-to-signal-line crosstalk conspicuous and also makes a precise tracking error signal unavailable.
Incidentally, the difference between the sum of signals A and C and that of signals B and D, that is, (A+C)-(B+D) has been well known to be usable as a focus error signal. The focus error signal is such that the (A+C) signal and the (B+D) signal are fed into the +input terminal and -input terminal of an operational amplifier, whereby not a phase difference but a difference in level therebetween is outputted as a focusing error signal. As the frequency area of the focusing error signal ranges from 10 to 20 kHz, the focusing error signal is obviously different in the frequency area from the tracking error signal of the phase difference system.